Vacuum in gas tank - already replaced almost everything

[Rocket Man]

Too bad you misunderstood the codes or the cap would have been the first thing replaced. BTW you can get the same codes if the cap isn't tightened enough. I've had them come on just because an attendant didn't tighten it. (Yes I live in Oregon where we sit in our vehicles while somebody pumps our gas which is especially nice in the winter)

 

[DonnieBrasco]

Too bad you misunderstood the codes or the cap would have been the first thing replaced. BTW you can get the same codes if the cap isn't tightened enough. I've had them come on just because an attendant didn't tighten it. (Yes I live in Oregon where we sit in our vehicles while somebody pumps our gas which is especially nice in the winter)

Yes indeed. It's a kick in the pants for sure. But, this Yukon is 15 years old with over 250k miles on it, so it definitely didn't hurt to replace all the other components.

 

[HiHoeSilver]

Too bad you misunderstood the codes or the cap would have been the first thing replaced. BTW you can get the same codes if the cap isn't tightened enough. I've had them come on just because an attendant didn't tighten it. (Yes I live in Oregon where we sit in our vehicles while somebody pumps our gas which is especially nice in the winter)

Didn't they recently change that?

 

[Rocket Man]

Didn't they recently change that?

Well... it seems there was a lot of stuff on FB making fun of us stupid Oregonians having to learn how to pump our own gas because the law changed. But in reality it just modified an existing law that let some stations in the very lightly populated counties (we have a lot of extremely lightly populated areas of the state) do self-serve so they don't have to pay an attendant when he/she might only see a handful customers in a day. They just expanded that law to more situations. But no, for the most part we still have attendants. And our gas is still cheaper than Washington to the north or California to the south. Some stations will let you do the actual filling if you have a custom rig but some are afraid they'll get in trouble. When I was driving my new bagged Silverado back from Cali a few weeks ago, I had one lady who absolutely would not let me fill it and she saw the Cali plates and wanted to explain the Oregon law to me. I told her I was from Oregon and just bought the truck but she was afraid her boss was watching. It was kind of funny. But with that truck, I don't usually have a problem doing the actual filling, but they get the pump ready and stand right there.

 

[DonnieBrasco]

Let us know if it works.

Welp. Didn't fix it. It still pulls a vacuum. Obviously it's worse the lower the fill is in the tank, e.g., when it's at a quarter tank or below, it gets worse. And it screws with my gas gauge and it drops quickly once it's under a 1/4 tank. I'm basically giving up on this. I've replaced every part I know that could be causing this. I may not have but another 6 months or so in this Yukon, so I guess I'll just live with this. But now I have other problems...

A little over a year ago, I started getting "reduced engine power" and throwing codes for the TPS. I replaced the TPS and didn't have any problems since. Until yesterday. Yesterday I picked up the truck after getting some new tires and got reduced engine power. I shut the car off and cranked it back up and it was gone. Then it happened again about 15 minutes later. Shut it off, cranked up and it was gone. Hasn't happened again since yesterday. But today I hooked up my scanner and pulled a bunch of throttle related codes: P0120, P0220, P1516, and P2135 (and also still throwing evap codes 443 and 455).

One possibility I am hoping: when I put the new TPS on a year or so ago, as you know it has about 8 wires to splice in. IIRC, I ran out of butt connectors and at least one wire was spliced either with a wire nut and tape or just twisting and tape. My first thought is to check all these connections. But my question is: could something as simple as one or two wires not having a good connection throw all those codes?? I'd really like to get it sorted out today so I can take it out of town this weekend. Thanks, y'all.

 

[Rocket Man]

First of all, I thought you understood the system isn't pulling a vacuum, it won't hold a vacuum. It has a vacuum leak. And as far as the TPS needing all it's wires connected to work or it will throw multiple codes: yes. One wire can actually keep the engine from running at all. As well as throw multiple codes. Also, I don't trust butt connectors in an environment where moisture could be present as in under the hood. And definitely not wire nuts. Corrosion can build up over time and the connections can become intermittent, and the hardest electrical problem to diagnose is an intermittent one because it might work fine while you're looking at it but later it starts failing. I would get a cheap soldering iron and some heat shrink tubing and solder/ heat shrink those wires in order to eliminate the possibility. Do one at a time so you don't get them mixed up, or do a diagram. But something like TPS wires need to be done right.

 

[Rocket Man]

When you said you replaced the "purge unit" near the gas tank, exactly what did you replace? The vent valve looks like this and is usually the problem:


The purge valve looks like this and can also be the problem:


If you haven't replaced them both, that's most likely your problem. Either one can cause those Evap codes.

 

[swathdiver]

...a bunch of throttle related codes: P0120, P0220, P1516, and P2135 (and also still throwing evap codes 443 and 455)

P1516 is your clue here. If the throttle plate is binding or sticks or doesn't work right, that will cause the other codes and the throttle body needs to be replaced. if the throttle plate works just fine, then the problem is your ECM.

P0443 refers to the Purge Solenoid. A Tech2 is needed to command it to open and close, so you can hear it click to see if it is working properly and to record freeze frame data. If this too is working properly after testing, the problem is also your ECM.

 

[DonnieBrasco]

First of all, I thought you understood the system isn't pulling a vacuum, it won't hold a vacuum. It has a vacuum leak. And as far as the TPS needing all it's wires connected to work or it will throw multiple codes: yes. One wire can actually keep the engine from running at all. As well as throw multiple codes. Also, I don't trust butt connectors in an environment where moisture could be present as in under the hood. And definitely not wire nuts. Corrosion can build up over time and the connections can become intermittent, and the hardest electrical problem to diagnose is an intermittent one because it might work fine while you're looking at it but later it starts failing. I would get a cheap soldering iron and some heat shrink tubing and solder/ heat shrink those wires in order to eliminate the possibility. Do one at a time so you don't get them mixed up, or do a diagram. But something like TPS wires need to be done right.

Sorry, the vacuum is still confusing me. I do understand it has a vacuum leak. I think I just chose a poor choice of words. I was referring to there being a massive pressure release when I unscrew the gas cap.

I'm leaving the Yukon home this weekend and when I get back on Sunday I'll get out my soldering iron and solder all those and see if I can get the code to clear. Thanks for the good advice. Any particular solder I should or should not use?

When you said you replaced the "purge unit" near the gas tank, exactly what did you replace? The vent valve looks like this and is usually the problem:


The purge valve looks like this and can also be the problem:


If you haven't replaced them both, that's most likely your problem. Either one can cause those Evap codes.

I've replaced both of those. The purge valve was under the hood on the block, the vent valve was underneath closer to the charcoal canister which I recently replaced as well.

 

[iamdub]

Pressure release when you open the cap is probably actually air rushing in which would mean the tank is under a vacuum. Unless there's something pressurizing the tank and the air is blowing out when you crack open the cap. At least you know the cap's seal is good! But, if there was a leak somewhere, then the tank wouldn't be able to hold air and pressurize- positively or negatively.

Even though you replaced the vent and purge solenoids, the circuit controlling them may be faulty (broken wire, loose terminal in side of a plug, etc.). The purge solenoid (on the motor) should steadily be pulsed with the engine running and the pulses should increase with RPM. With the engine running, unplug the vacuum hose going to it and put your finger over the nipple on the valve- you should be able to hear and feel the air being sucked into it. Unplug the electrical connection to it and see if the pulsing stops. If so, it's responding and likely operating as normal. The vent solenoid near the tank is controlled by the PCM to open when certain parameters are met. This is where a bi-directional scan tool and a simple cheap test light come in handy. It can command the vent solenoid to open for diagnostics. If the plug at the vent solenoid doesn't receive the signal when commanded (test light illuminating), then the circuit is open (broken) somewhere.